The present invention relates to a process for removing calcium from a hydrocarbon feedstock. It is in general terms a hydrodemetalation process using a fixed bed catalyst system, and, more particularly, comprises employing a layer of catalyst particles characterized as having a high volume percent of their pore volumes in the form of macropores, having a low surface area, and a low hydrogenation activity.
A few, but increasingly important, petroleum crude feedstocks, residua, and deasphalted oil derived from them, contain levels of calcium and sodium which render them difficult, if not impossible, to process using conventional refining techniques. The metals contaminants causing particular problems are in the form of generally oil-soluble organometallically-bound compounds including metal naphthenates. These species have been attributed to either naturally occurring metal complexes or solubilized metal compounds from recovery waters which comes in contact with crude oils. These compounds are not separated from the feedstock by normal processes, such as desalting, and in a conventional refining technique they can cause the very rapid deactivation of hydroprocessing catalysts. Examples of feedstocks demonstrating objectionably high levels of calcium compounds are crudes from China, such as Shengli No. 2, and those from the San Joaquin Valley in California, generally contained in a pipeline mixture referred to as San Joaquin Valley crude or residuum.
The problems presented by oil-soluble calcium and sodium in petroleum feedstocks and the necessity for their removal have only been recently appreciated, and the prior art contains relatively few references to its removal. U.S. Pat. No. 4,741,821, Hung et al. teaches a process for the use of a catalyst containing nickel to facilitate the calcium removal. U.S. Pat. No. 4,830,736, Hung et al. also teaches a similar system for the removal of both calcium and sodium. U.S. Pat. No. 4,744,888 discloses a catalyst system which facilitates sodium removal.
U.S. Pat. Nos. 4,853,109, 4,778,589, 4,778,591, 4,789,463, 4,778,590, 4,778,592 and U.S. Ser. Nos. 222,472, and 239,152, all commonly assigned to the assignee of the present invention, disclose various sequestering agents including aminocarboxylic acids, hydoxocarboxylic acids, dibasic carboxylic acids, carbonic acid, monobasic carboxylic acids, sulfuric acid, and their salts, are used for the aqueous extraction of nonporphyrin organometallic contaminants from hydrocarbonaceous feedstocks. The disclosures of all the aforementioned patents and applications are incorporated herein by reference.
In recent years, workers in the field of metals removal have developed hydrodemetalation (HDM) catalysts to protect the more active hydrodesulfurization, hydrodenitrification, or hydrocracking catalysts. Generally, the HDM catalyst contacts the contaminated feed and the metals are deposited before the feed continues through the catalyst bed contacting the active catalysts. In particular, complicated schemes of grading varieties of catalysts which differ in pore size, support composition, and metals loading can result in more efficient use of the individual catalysts.
Most grading schemes involve contacting the hydrocarbon feedstock with catalyst having large pores designed for metals capacity followed by catalysts with smaller pores and more catalytic metals to remove sulfur and other organic metals. In this way the contaminated feed initially contacts a less active catalyst, thereby allowing the feed to penetrate the catalyst more fully before metal deposition occurs. As the less contaminated feed continues through the catalyst bed, it contacts more active catalysts which promote the deposition of sulfur and other organic metals. Thus, for any given feedstock containing metals that penetrate to the interior of the catalyst, such as nickel and vanadium, there will be an ideal grading of catalyst which will result in the most efficient use of these catalysts from the top of the reactor to the bottom.
Workers in the field encounter a more complex problem when metals such as calcium or iron are present as in an oil-soluble form. In contrast to nickel and vanadium which deposit near the external surface of the catalyst particles, these metals can deposit preferentially in the interstices, i.e., void volume, among the catalyst particles, particularly at the top of the hydrogenation catalyst bed. This results in drastic increases in pressure drop through the bed and effectively plugs the reactor.
Conventional processes, which remove nickel, vanadium, and iron, generally have decreasing macroporosity and increasing mesoporosity in the direction of feed flow through the graded bed. The term "macropore" is used in the art and is used herein to mean catalyst pores or channels or openings in the catalyst particles greater than about 1000 Angstrom in diameter. Such pores are generally irregular in shape and pore diameters are used to give only an approximation of the size of the pore openings. The term "mesopore" is used in the art and used herein to mean pores having an opening of less than 1000 Angstrom in diameter. Mesopores are, however, within the range of less than 1000 Angstrom in diameter.
Previous workers found macroporosity to be strongly related to the capacity of catalyst particles to retain heavy metals removed from contaminated hydrocarbon feed. In the following catalyst zones, they prefer predominantly mesoporous catalysts. They found these catalysts to have substantially higher catalytic activity for hydrogenation compared to catalysts having lower surface areas and substantially a macroporous structure. Thus, they exploited these two phenomena to remove heavy metals from heavy feedstocks in a graded catalyst system.
In general, we have found that calcium deposits preferentially in the void volume among the catalyst particles. This greatly increases pressure drop through the bed and results in enormous reactor inefficiencies. In addition, we have found that sodium surprisingly behaves in a manner unlike any other metal encountered thus far. In particular, it deeply penetrates the catalyst particles. So the calcium deposits increase the pressure drop through the catalyst bed while the sodium works to block the active sites within the catalyst particles and deactivates them. As a result of our work, it has become clear that we cannot use conventional graded systems successfully to remove calcium and sodium from a hydrocarbon feedstock containing both of these metals. Thus, it is necessary for us to devise a graded catalyst system, taking into consideration such factors as shape, size, porosity, and surface activity of the catalyst particles that successfully removes both calcium and sodium from the hydrocarbon feedstock. It is an objective of our invention to provide a catalyst system for removing calcium from a hydrocarbon feedstock. It would be advantageous if our catalyst system also removed oil soluble sodium compounds.